Chromium plating process and bath



Aug. 23, 1960 A. A. .JOHNSON ET AL 1 2,950,234

CHROMIUM PLATIN@ PROCESS ANDBATH Filed Feb. 2e, 1958 300 400 500 azw/ssoa/f cf ai, @MS/U75@ CHROMIUM PLATING PRocEss AND BATH Andy AlbertJohnson, Oak Park, and George Dubpernell, Huntington Woods, Mich.,assignors to Metal & Thermit Corporation, Woodbridge Township, NJ., acorporation of New Jersey Filed Feb. 26, 1958, Ser. No. 717,666

13 Claims. (Cl. 204-51) This invention relates to chromium plating witha CrO3-containing plating bath having sulfate and silicouoride as mixedand cooperating catalysts which saturate the bath and whoseconcentrations are self-regulated.

More particularly, a portion of the total CrO3'content is supplied bypotassium dichromate, which also saturates the bath, so that theconcentration of said portion of CrO3 is self-regulated. Theconcentrations of the bath ingredients arethus self-regulated to a highdegree. The bath is unusually constant in quantitative and qualitativecomposition over a substantial range of plating conditions, freeing theoperator to a greater extent than heretofore possible of the necessityof closely watching the bath balance and of making frequent additions tomaintain or correct the bath.

Over a substantial portion of its range of plating conditions the bathis characterized by having a ratio of CrO3 to catalyst that issubstantially constant, that is, it is substantially unaiected bychanges in temperature and concentrations of CrO3 and catalyst.

The bath is further characterized by exhibiting a substantially reducedtendency to etch an article being plated and by producing chromium plateof improved clarity and brightness, these effects being particularlyapparent in the low current density plating range. Other advantages willappear hereinafter.

Generally speaking, the bath solution comprises chromic anhydride (CrO3per se), two catalyst-supplying compounds, namely, strontium sulfate andpotassium silicofluoride, each having a limited solubility in the bathsolution, and potassium dichromate. The strontium sulfate and potassiumsilicouoride are each present in an amount sucient to saturate the bathwith dissolved catalytic sulfate and silicotluoride and to providetherein an undissolved residue of each compound. The potassiumdichromate has several purposes or effects, one of which is to controlthe concentration of the dissolved silicouoride in the bath bysuppressing such concentration, the dissolved potassium dichromateacting by the common ion effect to decrease the solubility of thepotassium silicoiluoride. The concentrations of the dissolved sulfateand silicofiuoride are the saturation concentrations, and both areself-regulated owing to the presence of undissolved residues ofstrontium sulfate and potassium silicofiuoride in equilibrium therewith.The sulfate concentration, however, is normally the unsuppressedsaturation concentration, while the silicoiluoride has a suppressedsaturation concentration.

While the use of strontium sulfate and potassium silicoiluoride insaturating amounts is taught in U.S. Patents Nos. 2,640,021 and2,640,022, and the suppression of the silicouoride is taught in No.2,640,022, these patents do not disclose the use of saturationconcentrations of potassium dichromate and the eiects thereof, asdescribed below.

The potassium dichromate is present in an amount sutlcient to saturatethe bath and to provide therein an undissolved residue in equilibriumtherewith; it pronited States Patent ice vides concentrations ofdissolved potassium and dissolved CrO3 which not only saturate the bathbut which are self-regulated. As described, the `dissolved potassium isresponsible for decreasing the solubility of the potassiumsilicouoride', i.e., suppressing the concentration of the dissolvedsilicofluoride; while the dissolved CrO3 constitutes a substantialportion of the total dissolved CIOS in the bath. The saturationconcentration of the dissolved potassium has been found to beparticularly suitable for controlling the concentration of thesilicotluoride within desired limits.

As may therefore be apparent, the concentrations of the dissolvedsulfate and silicouoride are self-regulated, that of silicofluoride issuppressed by dissolved potassium whose concentration is alsoself-regulated, and the concentration of dissolved CrO3 supplied bypotassium i dichromate is self-regulated, which in eiect means that solong as undissolved dichromate is present, the total dissolved CrOsconcentration is self-regulated. These automatically controlledconcentrations mean that the bath is self-adjusting for drag-out of allessential ingredients: catalysts, CrO3, and suppressor, providing a bathof unusual stability having wide application in chromium plating,particularly in barrel plating where high dragout losses areencountered, and where, too, owing to the improved tolerance of the bathto concentration changes, less frequent correction is required. Theseadvantages are generally realized in other applications of the bath.

The bath is frequently designated a chromic acid chromium plating bath,but since its chromium content is supplied by at least one othercompound besides chromic acid (more accurately called chromicanhydride), the bath is referred to herein as a CrOa-containing bath.Its CrOa content is supplied by CIOS per se, usually referred to aschromic anhydride or chromic acid, and by the amount of CIOS present inthe dissolved potassium dichromate.

Numerically, the total concentration of dissolved CrOa in the bathsolution may be about 150 to about 700 g./l. The concentration generallyincreases with the temperature, which may vary from room temperature tothe boiling point of the solution, more particularly from to 170 F. Ofthe total dissolved CIOS, about l0 to about 90% by weight is supplied bypotassium dichromate, and the balance is supplied as chromic anhydride.These quantities vary with respect to one another in the manner dened bythe area ABCDA of the graph shown in the accompanying drawing. It isapparent from the graph that total dissolved (lr03 increases withincreasing chromic anhydride; at a given chromic anhydrideconcentration, and as temperature increases, total dissolved CrOaincreases with increasing CrO3 supplied by dichromate; and Cr03 fromdichromate increases with increasing temperature and decreases withincreasing chromic anhydride. As also illustrated by the graph, thechromic anhydride ranges from 50 to 500 g./l.

The upper limit of 700 g./l. for the total -dissolved CrO3 .is apreferred limit, as this quantity may range to 800 g./l. and more,provision for operation at the latter concentration levels being made byincreasing the chrornic anhydride concentration and/or increasing thetemperature, and in such case it will be understood that for the overallrange of to 800 g./l. the area ABCDA will substantially define therelationships of the variables as before except that the line BC and/orthe line DC Y range is 10 to 90%.

' undissolved residue of the same.

3 Y weight of the total dissolved CrO3, the balance of the latter beingsupplied by chromic anhydride; in this event it will be apparent thatquantities of chron-lic anhydride Vless than 50 `g./l.,` and going down,say, to,about.5 g./l.,

willsulice.Y The broken or dash line JK identiiies the limit of 99% byweight CrG3 from dichromate, and the temperature lines are extended toit by broken lines. Thus, the overall range of dissolved CrO3 fromdichromate is about to 99% by weight, and the preferred Operation ispossible in the area ADKJA, but the more preferred'area-is ABCDA. K

VAsrnoted, the amount of the potassium ydichromate is `always sufficientto saturate the bath and to produce an Preferably thedichromate is addedto the bath per se; it may also be formed in situ in the bath byreaction between a bath-soluble CrOS-containing compound and aybath-soluble potassium compound of a non-catalytic anion, Vthesecompounds being supplied in amounts that will provide potassiumdichromate in excess of its solubility in the bath. The bath-solubleCrOB-containing compound is selected from thegroup consisting of chromicanhydride and suitable CrOa-contaim'ng compounds of a non-catalyticcation, such for example as chromic acid (HzCrOg)V in solution,4dichromic acid (H2Cr207) in solution, etc. The bath soluble potassiumcompound ,of a non-catalytic ,anionmay be potassium carbonate, potassiumbicarbonate, potassium hydroxide, potassium chromate, etc.

Illustrative of the amount of potassium dichromate to be added to thebath solution is the observation that its saturation-solubility at 110F.7in each of five aqueous solutions having dissolved therein,respectively, 100, 200, 300, 500, vand 700 g./l. of chromic anhydride,is respectivelyv about 199, 195,Y 191, 178, and 147 g./l. 'I'he amountof dichromate to be added to such solutions at 110,` F., therefore, willalways exceed these saturation solubilities so as to produce anundissolved residue of theadichromate in the solutions. The dichromatesolu- V bility increases with rising temperature, and vice Versa, .asindicated by the graph, so that this consideration must also be takeninto account. These facts also apply when theY dichromate is Vformed insitu.

For the broad ranges of OrO3 and temperature noted .Vaboveand asdened bythe area ABCDA of the graph,

theconcentration of dissolved sulfate is the saturation concentrationand that of dissolved silicouoride is the suppressedV lsaturationconcentration. Numerically, the dissolved Ysulfate may Vary from 1.16 to2.60 g./l. and the dissolved silicofiuoride from 0 .25 to 1.81 g./l.Total catalyst varies from 1.5 to 4.5 g./l. It may be noted that theconcentrations of both catalysts are low, particularly that of thesilicofluoride. The low silicouoride concentration is believed tocontribute to the reduced etching tendency of the bath, and, togetherwith the low proportion of silicouoride to sulfate, is believed tocontribute to the improved clarity and brightness ofl plate used fordecorative applications.

l TheV sulfate and silicoiluoride are supplied in the form of strontiumsulfate and potassium silicofluoride, as stated. The potassiumsilicouoride is preferably provided per se. It may also be formed insitu in theV bath by reaction between a bath-solublesilicouoride-containing compound of a non-catalytic cation and abath-soluble potassium compound of a noncatalytic anion, forexample bythe reaction between hydrouosilicicacid or zinc silicolluoride andpotassium carbonate. Similarly,

Ystrontium sulfate may be formed in situ but it is Vpreferred to add itper se to the bath. Formation in situ the Cros/catalyst ratio is 120:1to 130:1.

Yrange of :1 to 100:1.

0.22 to 0.55. With this bath it has been found that Vthe ratio of totaldissolv'edrCrOz, Vto the sum of dissolved s'ulfate plus 1/2 thedissolved silicouoride, hereinafter referred to as the Cr03/ catalystratio, is substantially independent of changes in temperature and CrO3and cat- :alyst concentrations. .The ratio liesmin the range of 115 :1to 140:1, and from a plating point of view a ratio that varies withinthis narrow range represents a substantially constant ratio. By virtueof its constant ratio, this preferred bath is well adapted to barrelplating.

The area on the graph designated by the arrow L rep resents anlespecially preferred bath Vsolution operable at about YF. having atotal dissolved CrO3 concentration of about 250 to about 260 g./l. about50 to 55% of the latter being' supplied by potassium dichromate andabout g./l. being supplied as chromic anhydride.

VThe total dissolved sulfate is about 1.68 to 1.94 g./l., the

dissolved suppressed silicofluoride is 0.52 to 0.57 g./1., thesilicouoride-sulfate proportion is'0.29 to 0.31, and This bathcomposition is (specially suitable for commercial operation Abecause itemploys the temperature and Cr03 concentration ranges most normallyused, and because of the constancy of the ratio, the latter having aValue particulrly adapted VVfor barrel chromium plating.

In some 'cases the Cr03/ catalyst ratio may, if desired, be varied bythe addition of a small amount of soluble `strontium or soluble sulfatewhile still maintaining the sulfate concentration of the bath saturatedand self-regulated. vrAddition of strontium increases the ratio bysuppressing dissolved Vsulfate; and addition of sulfate decreases theratio by providing additional dissolved sulfate. In both cases vtheaddition can'be made so asto maintain the total dissolved sulfateconcentration of the bath Vboth saturated and self-regulated. While theaddition of sulfate Vinvolves the precipitation of some strontium asstrontium sulfate, which may lead one to think that all Ythe addedsulfate will be so precipitated, actually it has been found thatVadditional dissolved sulfate Vmay be introduced to the bath Yso as todecrease the ratio. This procedure provides a convenient manner ofchanging the ratio, within limits, while retaining the property ofselfregulation. In particular, theprocedure is useful to extend theplating range of the bath. For example, by addition of sulfate, bathsmay be obtained having com positions more commonly used in hard chromiumplating, that is, having -a ratio range of 90:1 to 100:1, or a wider Asillustrated in Example 5, a bath having a ratio of 118:1 may be adjustedto a ratio of 92:1 or to a ratio of 221:1 in this way. The procedureapplies to all the baths described herein, particularly tothe preferredbaths. The compound to be added to the Vbath should be more solubletherein Vthan strontium sulfate and .should provide a soluble ionicVcomponent of strontium sulfate. Suitable strontium compounds includestrontium carbonate, strontium hydroxide, strontium chromate, strontiumoxide, and strontiumV bichromate. Suitable sulfate compounds are sodiumsulfate,

f ysulfuric acid, ammoniu'msulfate, calcium sulfate, lithium ofstrontium sulfateand potassium silicouoride is dev scribed in saidPatent No. 2,640,021.

A preferred bath solution having more restricted concentration's thanthose described is one having a total wir by Weighrvflhetotal dislvedois Supplied by (sulfate, 4chromic sulfate, Vsodium Ybisulfate, etc.

The amount ofthe A'sulfate compound can be as low as that 'p equivalentto0.1.g.'/l. strontium, and'on the other handit should'be less than'anA'amount which prevents the Vtotal dissolved sulfate from beingself-regulated. When resort -tothis proceiu'reishad, itis apparentthat'the sulfatecon- "entatio'n may 'bea'ltered, so that the procedurecohstitutes an exception to the general practice described above of notsuppressing or altering the sulfate.

Where it is not desired to change the ratio, the bath lis preferablymaintained h'ee of compounds acting to alter the concentration of thetotal dissolved sulfate.

The bath solution may be made' up by adding, to a given volume of water,chromic anhydride, potassium silicotiuoride, strontium sulfate, andpotassium dichromate, each in the desired amount. The ingredients arepreferably supplied in subdivided form, and stirring is used untilequilibrium is obtained. Varying bath compositions may be made up bysimply varying the amount of chromic anhydride and/or water, and anexisting bath solution may be varied by adding chromic anhydride orwater.

Current densities conventionally used in chromium plating are suitable,such for example as described in Chromium Plating, by Morisset et al.,published by Robert Draper Ltd., 1954, pages 351-363. Numerically, thecurrent density may range from 0.2 a.s.i. (amperes per square inch) to110 a.s.i., preferably from 0.2 to 6.0

Conventional chromium deposits of any type can be plated, includingdecorative plate, hard plate, crack-free plate, and porous plate. Thedeposits may have varying thicknesses ranging from a thickness justsuicient to cover the pores of the basis metal to any practical desiredthicker deposit for which a demand may exist.

Articles made of any of a variety of basis metals may be plated, such asplain carbon steels, alloy steels including stainless steel, iron, castiron, copper and copper alloys, nickel and nickel alloys, zinc and zincalloys, aluminum and aluminum alloys, tin and tin alloys, lead and leadalloys, etc. In general, the basis metal may be any metal, or anundercoat on a metal, that can be chromium plated.

The invention may be illustrated by the following examples. A briefexplanation of some of the terms and abbreviations used in the examplesmay be given: C A. is chromic anhydride; sulfate is S04; silicouoride isSiF; catalyst proportion or cat. prop. is SiF6SO4; and catalyst ratio orcat. ratio is CrOs/SO-'i-l/zSiI-is.

EXAMPLE l This example illustrates mixtures for making bath solutionsand the compositions of the latter, and it com- I pares them with priorart mixtures and bath solutions.

A solid, inely divided mixture was made up consisting of 35.8% chromicanhydride, 59.7% potassium dichromate, 1.5% potassium silicouoride, and3.0% strontium sulfate, all on a Weight basis. Two parts of the mixturewere taken to prepare two aqueous solutions, Nos. 1 and 2. Theequilibrium compositions of No. 1 at 90% F., and No. 2 at 110 F., aregiven below in Table l.

Another solid, finely divided mixture was made up consisting of 27.6%chromic anhydride, 69% potassium dichromate, 1.1% potassiumsilicouoride, and 2.3% strontium sulfate, weight basis. An aqueoussolution, No. 3, was prepared with this mixture, and its equilibriumcomposition at 130 F. is given below.

Two other solid, nely divided mixtures each consisting of chromicanhydride, potassium dichromate, strontium chromate, potassiumsilicofluoride, and strontium sulfate, were added to water to provideNos. P-l and P-2, both of which were prior art commercial solutionshaving equilibrium compositions at 130 F. as noted in Table l.

Of these solutions, Nos. 1, 2, and 3 illustrate the invention; in eachof them potassium dichromate was present in excess of its solubility,the excess being greatest in No. 1 and least in No. 2; in Nos. P-l andP-2, the prior art solutions, no excess potassium dichromate waspresent. All solutions were comparable to the extent that the catalystswere suplied by potassium silicouoride and strontium sulfate andhad'saturated, self-regulated concentrations.

Table 1 S0111. No 1 2 3 P-l P-2 Temp., F 90 110 130 130 13o TotalDissolved CrOg, 205 247 302 225 175 ohr0m1cAnhydride,g./1 120 12o 120205 155 OrO; from Potassium Dichromate,percent of Total. 41.4 51.5 60.49.1 13.2 Cat. Ratio 124:1 130:1 125:1 84:1 89:1

l EXAMPLE 2 The etching action of a bath solution of the invention,corresponding to No. 3 of Example 1, is compared here with. that of twoprior art bath solutions, corresponding to Nos. P-1 and P-2 ofExample 1. vAll baths were operated at 130 F. In each case, a weighedunetched steel strip, having an exposed area of 6 sq. in. was used ascathode, and a low current density was applied for 1 hour, after whichthe strip was removed, washed, dried, and weighed to determine the lossin weight. Tests were run throughout the current kdensity range ofmaximum etch (up to 0.05 a.s.i.) and also up to 0.10 a.s.i.Substantially identical, unetched steel strips were used for all runs.The results are tabulated in Table 2, the loss of weight of the stripsbeing reported as grams per hour of applied current per 6 square inchesof metal surface.

Table 2 Loss of Current Weight o! Soin. No. Density, Metal, a.s.i.g./hr./6

sq. in.

0. 034 0.0375 3 .05 .05 ,05 .0625 .10 .069 a: at Pl '.05 '.350 .1o .3815sa ai P2 '.05 i255 .10 269 As is apparent, the etching produced bysolution No. 3 is substantially less than that produced by the other twosolutions; at 0.05 a.s.i., for example, the etching was reduced to only14% to 24% of that in the other solutions.

EXAMPLE 3 This example demonstrates the reduced tendency of the catalystratio to change with changes in temperature. Three bath solutions, Nos.4, P-3 and P-4 were made up each having a total dissolved CrO3 contentof 200 g./l. at F. Solution No. 4 had chromic anhydride and excessamounts of potassium dichromate, strontium sulfate, and potassiumsilicouoride; its composition was within the limits of the invention,and it was prepared in a manner similar to Nos. 1, 2 and 3 of Example 1.Nos. P-3 and P-4 were prior art commercially used bath solutions, hadexcess amounts of strontium sulfate and potassium silicofluoride but notof potassium dichromate, and were prepared from mixtures like those usedfor Nos. P-1 and P-2 of Example 1. The catalyst ratio was determined foreach solution at 90, 110 and 130 F. and is given in Table 3. The totalCrO3 concentration for Nos. P-3 and P-4 remained at 200 g./l. at eachtemperature, but that for No. 4l increased to 250 g./l. at and to300g/l. at 130.

7 It is evident that the ratio for No. 4 was substantially constantthroughoutthe temperature interval while that for lthe other solutionsvaried considerably.V g

n n l' Y XMPLE 4V Y gandnoted below in Table 5, represents that of theoriginal solution. To No. 7/t'here was added 1.0 g./l. of stronmExample'il j ,7 wasthen divided into Vtive equal parts to 5 tiuin inthe forni ofstrontium carbonate, and similarly, v The reduced tendency of thecatalyst ratio to change 3.0 g./1. of `stroiitiumvvas added to No. 6. ToNos. 9 with changes in total dissolved CrO3 concentration 1s and 10wereiadded O Sand 1.0 g /1 respective1y of SUL ShOWIl here Three batheSolntlonSfNm- P3 and P-S fate, added as 4sodium sulfate. The compositionof each were used, each having a total dissolved CrO content oiiSolution was Vgleteriniimd at 110 `and is Set forth in 200 g./1. at 90F. Solution'No'. 5, illustrating the 1n- 10 Table5 A vention, wasprepared similarly to Nos. 1, 2 and 3 of Table 5 ExampleV l. SolutionNo. P-3 corresponds to No. P-3 of Example 3, while No. P-5 was acommercially `Used m'Nmr 6 7 8 9 10 bath solutio'n similar to Nos. P-1and P-2 of Examplel except for its higherrratio.V The catalystratio wasde- 5 strontium added g /1 3 0 1 0 termined for each solution in theas-prepared state or sulfate added,g. n 0.5 1.0 low CrQ concentrationlevel, and also atter additions gfstleg Effi inl/ 1: O Zg L22 1 2g figfig of chromic anhydride to produce medium and high siuconuoride, g./10.53 0.57 0.57 0.56 0. 57 concentradas- The 4441401344 apparent fromT43@ 5; 3.3 .33 .33 3.3 33 4, but it may be noted here that for No. 5the addition 20 to the low concentration solution was g./l. and that tothe medium concentration solution was 34 g./l.; for No. As 1S apparentthe fano increases, Wlth ih e addmon of P 3 the cdrresponding additionswere 50 g /1 and 50 soluble strontium and decreases with addition ofsoluble g./l.; Vand for No. P-5 the additions were 25 g./1 and sulfate.If'the'ratios are plotted as ordinates against the 25g/l. Alldeterminationswere made at 110 F. The 25 strontium additions asabscissas, and if the sulfate addif .terms 510W?, medium, and high arere1ative being tions are expressedA stroichiometrically as the negativeused for convenience in identifying the concentrations. 'equivalentofthe strontium additions S0 that a continuous Y Y abseissa is presentranging from about minus 1.0 g./l. T ble 4 of added strontium through 0g./i. and up .i6 3.0 g./i., it 3 0 willbe found that acontinuous curveis obtained. This LOW 001ml. High 001ML curve is essentially linear inthis range and shows that soil N0 foreach 0.3 g./1. increase or decreasein added strontium', Cro., Cat. Cros, Cat. oros, oat, 'the ratiorespectively increases or decreases by approxig./i. Ratio g./1. Ratiog./1. Ratio mately 10 units. Y 35. EXAMPLE 6 33 131 312i 3i 3421 175AY11221 Y200 13621 225. ..16221 VThe eiect of temperature as well astotal dissolved Y 'Cr03 and chroriiic anhydride concentration,are'demon- As shown, the ratio for No. 5 was substantially constantStrated herein- SeYeral 'Solutions of Varying ohrorn'io nnthroughout theaddition's'while'that for'No's. P-'3 and 40 h'dnde Concentration WerePrepared, eqn'lhoraed at 90p P5 varied considerably, F. for severalhours, and then analyzed. Similarly, other solutions Wereprepared andanalyzed at progressively EXAMPLE 5 higher temperature levels. TheY:analyses of these solu- There is illustrated in this example themanner in which tions, numbered from 1'1 to 2 8, are shown in Table 6.

Table 6 I Total CrOg from Siloco- Soln. Temp, C.A., Dissolved Pot.Sulfate, Fluoride, Cat. Cat.

No. F g./1. CrO3,g./l. Dihrom., g. g.ll. Prop. Ratio percent 50 153'67.4 1.16 0.25 0.22 119 90 70 167 53.0 1.27 0. 29 0.23 113 90 120 20541.4 1.46 0.37 0.25 124 90 170 243 31.5 1.56 0. 41 0.26 141 00 220 23021.4 1.63 0.61 0.36 133 90 320 330 15.3 1.63 0.53 0.36 132 v120 26454.5 1. 34 0.57 0. 29 116 110 220 346 36.5 2. 09` 0.66 0.32 143 110 320`445 23.0 2.01 0. 60 0.34 133 120 306 60.3 2.06 0.73 0.36 126 130 220 33543.0 2.26 0.34 0.37 144 130 320 479 33.2 2. 23 0. 90 0.40 173 120 37663.2 2.24 1.03 0.43 135 150 220 464 52.7 2.43 1.23 Y 0,45 1.50 150 320555 42.5 2.41 1.17 0.43 135 120 456 73.5 2.25 1.31 0.30 145 170 220 52653.2 2.56 1.63 0.64 156 170 Y320 605 47.2 2.55 1.76 0. 69 177 Vthecatalyst ratiomay be varied by the addition of soluble 35l Besides therelationships apparent h'om the graph, sevstrontium or soluble vsulfatewhile still maintaining the eral others of Vbroad applicabilityinvolving dissolved bath bath saturated and self-,regulated with respectto the sulingredients, temperature, and other factors may be noted fateconcentration. in connection with Example 6. 5

A "bath solution was made 'up containing 120 g./1. As the temperatureincreases, the total catalyst concenchromic anhydride,200g/lf-potassium-dichromate, l5 70 tration increases, including boththe sulfate and silico- 'g./1.` potassium silic'ouoiide, and 10g/Lstmntiiiinn sulfluoride. Through .the .temperature range of 90 to 170fate. The last three compounds Were present in excess V1r.,'tota1catalyst may increase from about 1.5 to about of their solubilities.`YTheY solution,` 'which may be desig- 4.5 g./l., sulfate increases fromabout 1.16 to 2.60 g./l., nated the original solution (it correspondslto No. 2 of silicouoride from about 0.25 -to 1.81 g./l., and the silico-,75 liuoride-sulfate proportion from 0.22 to 0.8.0. A

With increasing total dissolved Cr03 from 150 to 605 g./l., or to thehigher concentration of 800 g./l., total catalyst, sulfate,silicouoride, and the silicouoride-sulfate proportion gradually increasewithin the general limits noted in the preceding paragraph.-

As chromic anhydride increases in the rangernoted in Table 6, `or in theWider range of 50 to 500 g./l., total catalyst increases gradually untilthe anhydride has a value of about 320-350 g./l. and beyond that tendsto remain constant or decline a little. Sulfate and silicouoride eachincreases to an anhydride value of about 220-350 g./l. and then eachdecreases, the silicouoride increase and decrease tending to be moregradual than sulfate. The silicoiiuoride-sulfate proportion tends to befairly constant. The general limits of these various catalyst quantitiesare as noted above.

Catalyst ratio generally increases with `increasing total dissolved CrO3concentration, and as indicated in Table 6, the increase in ratio tendsto be greater at the lower temperatures. At a given temperature, theratio increases with chromic anhydride and With total dissolved CrO3. Ata chromic anhydride concentration level of 120 g./l., the ratioincreases with temperature, and this is true at a concentration level of220 g./l., the ratio values for both levels varying between 119:1 and156:1, which, as indicated, represents a substantially constant ratiofrom a plating point of view; at 320 g./1. the ratio decreases from192:1 to 177:1 as temperature increases; it is thus apparent thatintermediate the 220 and 320 g./l. levels the ratio is exactly constantas temperature increases. Such exact constancy of ratio with increasingtemperature is unique as well as useful.

The preferred ratios have been described above as extending up to about140:1; these ratios are not only Well adapted for barrel plating butalso they favor the plating of crack-free deposits, and porous chromiumdeposits. In some cases a ratio as high as 200:1 is useful, as forplating chromium on copper, for example; and ratios up to about 250:1are applicable with some ybasis metals such as copper, brass, yor zinc.In general the useful ratios are those below 250:1, and as will beunderstood, the baths described herein will be made up and operated atsuch ratios. At the other end, ratios of about 100:1 and up arepractical, as are those below about 100:1, obtainable by addition of asoluble sulfate-containing compound as described.

The baths not only have the advantages of prior baths of self-regulatedcatalyst contents, such as good current etiiciency and plating speed,wide plating range, good covering power, activating eect in plating overpassive nickel, leveling action, and smooth deposits, but they alsoprovide control of the Cr03 and suppressor potassium contents owing tothe presence of excess potassium dichromate. The control of thecatalyst, CrO3 and suppressor potassium concentrations is inherentbecause as long as excess ingredients are present, these concentrationsare self-regulated by virtue of the solubility characteristics of theingredients. The baths are simple in composition, thus at a giventemperature there are only two variable component concentrations, waterand chromic anhydride; and they are simple in operation, behaving-muchlike a solution of one ingredient in their stability to temperature andconcentration changes, because from a chromium plating point of view,apart from the temperature variable,

rthere is only one composition variable, the chromic anhydride.

The baths are quite appropriate for -barrel plating, where drag-ont isIa special problem, because of the insensitivity of the bath compositionto temperature and concentration changes. The usefulness of the baths inapplications where drag-in is present has been noted; drag-in may occurin several Ways, as in plating processes that inclu'de a sulfuric acidtreatment, or where the parts -to be plated are of `a type characterizedby high drag-in. A somewhat reduced spray loss is realized by the use ofexcess amounts 10 of potassium dichromate. An advantageous applicationof the mixture used -to prepare the baths is as an addition agent forcertain existing baths to help produce brighter and clearer plate,particularly in 'low current density plating ranges.

In the light of the foregoing description, the following is claimed:

1. An improved method of electrodepositing chromium on a metal articleby means of -an aqueous CrO3-containing, catalyst-containing, chromiumplating bath, said method being characterized by having a CIOS/catalystratio in said bath which has a substantially :reduced tendency to changewith changes in CrO3 and catalyst concentrations and temperatures, whichcomprises essentially: passing current from lan anode to said article asa cathode immersed in said bath at a temperature of 90 to 140 F.; saidbath containing a total of dissolved CrO3 of about 150 to about 380g./l., strontium sulfate and potassium silicouoride ascatalyst-supplying compounds each in an amount sufficient to saturatesaid bath with dissolved catalytic sulfate and silicouoride,respectively, an'd to provide therein an undissolved residue of eachcompound, and potassium dichromate as a non-catalytic,silicouoride-suppressing, Cr03- Iand potassium-supplying compound; saidpotassium dichromate being present in an amount in excess of itssolubility in the ba-th and provid ing: (l) an undissolved residuethereof in sai'd bath, (2) a portion of said total dissolved Cr03 equalto about 30 to about 77 by Weight of said total, and (3) an amount ofdissolved potassium to suppress the concentration of the dissolvedsilicofluoride; the concentration of said portion of dissolved CrO3being self-regulated owing to the presence of said undissolved residueof potassium dichromate, said suppressed silicoiluoride concentrationbeing selfregulated owing to the presence of said undissolved residue ofpotassium silicotluoride, said dissolved potassium concentration beingself-regulated by virtue of the presence of said undissolved residues ofpotassium silicouoride and potassium dichromate, and said dissolvedsulfate concentration being self-regulated owing to the presence of saidundissolved residue of strontium sulfate; 70 to 170 g./l. of said totaldissolved CrO3 being supplied as chromic anhydride; the relationshipbetween said temperature, said chromic anhydride, said ftotal and saidportion of dissolved CrO3 being variable in the manner dened by the areaEFGHE of the graph shown in the accompanying drawing; and the ratio oftotal dissolved CrO3 to the sum of dissolved sulfate plus 1/2 thedissolved silicouoride being in the range of :1 to 140:1.

2. The method of claim 1 wherein said dissolved suppressedsilicoiluoride concentration is about 0.30 to about 1.20 g./l., saiddissolved sulfate concentration is about 1.25 to about 2.40 g./1., andwherein the proportion of dissolved suppressed silicofluoride todissolved sulfate is in the range of 0.22 to 0.55 and increases in saidrange with increasing temperature.

3. The method of claim 1 wherein said CrOa/catalyst ratio is decreasedbelow 115:1 by adding a soluble sulfate compound "to the bath in anamount to maintain the total dissolved sulfate concentration bothsaturated and selfregulated, said soluble sulfate compound having agreater solubility in the bath than said strontium sulfate.

4. The method of claim 1 wherein said CrOs/catalyst ratio is increasedabove :1 by adding a soluble strontium compound to the bath to suppressthe dissolved sulfate concentration, the amount of said solublestrontium compound being such as to maintain the total dissolved sulfateconcentration both saturated and self-regulated, said soluble strontiumcompound having a greater solubility in the bath than said strontiumsulfate.

5. The method of claim 1 wherein said bath is free of compounds actingto alter 'the concentration of dissolved sulfate.

6. An improved method of electrodepositing chromium on a metal articleby means of an aqueous CrOg-contaiu- 11 ing catalyst-containing,chromium plating bathhsaid rnethod being characterized'by having aCrOa/catalyst ratio in said bath which has a .substantiallyY reducedtendency to'change with lchanges inCrOg concentration and temperature,which comprises essentially passing current from an anode to saidarticle as a Vcathode immersed in said bath at a temperature between.room temperature and the boiling point of the bath. solution; Ysaidbath containing dissolved CrO3 and, strontium sulfate and potassiumsilicouoride as catalyst-supplying compounds each in an amount suicientto saturate said bath with dissolved catalytic sulfate and silicouoriderespectively and to provide therein an undissolved residue of eachcompound, and potassium dichromate as a noncatalytic,silicoiluoridesuppressing,l Cr03- and potassiumsupplying compound; saidpotassium. dichromate being .dissolved CrO3 being self-regulated owingto the presence of said undissolved residue of potassiumrdichromate,Vand said dissolved sulfate concentration being self-regulated :owing tothe presence of said undissolved residue of .strontium sulfate; thebalance of said dissolved CrO being supplied as chromic anhydride; r

7. The method of claim 6 wherein said CrO3/catalyst ratio Vis decreasedby adding a soluble sulfate compound Vto the bath in an amount tomaintain the total dissolved sulfate concentration both saturated andself-regulated, said soluble sulfate compound having a greatersolubility in the bath than said strontium sulfate. i r

'8. The method of claim 6 wherein said CrO3/catalyst .ratio is increasedby adding a soluble strontium compound to the bath to suppress thedissolved sulfate concentration,

the amount of said soluble strontium compound being such as to maintainthe total dissolved sulfate concentra-v tion both saturated landself-regulated, said soluble strontium compound having a greatersolubility in the Ybath than said strontium sulfate.

-Y 9. An improved method of electrodepositing chromium on a metalarticle by means of an aqueous CrOa-containing, chromium plating bath,which comprises essentially: passing current from an anode to saidarticle as a ,cathode immersed in said bath at a temperature of 90 to170 F.; said bath containing a total of dissolved CrO3 of about 150 toabout 700 g./l., strontium sulfate and potassium silicouoride ascatalyst-supplying compounds each in an amount sufficient to saturatesaid bath Y Vwith dissolved Icatalytic sulfate and silicoiiuoride,respectively, and to provide therein an undissolved residue of eachcompound, and potassium dchromate; said potassium dichromate beingpresent in an amount in excess of its solubility in the bath andproviding: (1) an undissolved residue thereof in said bath, (2) aportion of said total dissolved CrO3 equal to about 10 to about 90% byweight of said total, and (3) an amountof dissolved potassium tosuppress the concentration of the dissolved silicouoride; saidsuppressed silicofluoride anddissolved potassium concentrations beingVself-regulated by virtue of the presence of said undissolved residuesof potassium silicoiiuoride and potassium Vdichromate, the concentrationof said portion of dissolved'CrOs being self-regulatedowing.tothepresencef of said lundissolved, residue of potassiumdichrornate, vand saidV dissolved ,sulfatecon-- centration'bengVself-regulated owing to the presence of said undissolved` residue ofstrontium '.s'ultate; 50` to 500 g./l. of said totalV dissolved CrO3being supplied as chromioanhydr'ide; the relationship between thetemperature, the total and said portion of dissolved CrO3, and thechromic anhydride being variable in the. manner deiined by the areaABCDA of the graph shown in the accompanying drawing; and the ratio oftotal dissolved Cr03 to the sum of dissolved sulfate Vplus 1/2 thedissolved silicouoride being below about 25 0: 1.

10. An improved method-of electrodepositing chromium on a metal articleby means of'an Aaqueous Cr03- containing, chromium plating bath, whichcomprises essen tially: passing current from an anode to said article'as a cathode immersed in saidbath at a temperature of F. to the boilingVpoint of the bathlsolution; said bath containing artotal of dissolvedCrO3 of about 150 to about 800 g./1., strontium sulfate and potassiumsilicouoride as catalyst-supplying compounds each in an amount suic'ientvto saturate said bath with dissolved catalytic sulfate andsilicoiiuoride, respectively, and to provide therein an undissolvedresidue of each compound, and. potassium dichromate; saidpotassiumdichromate being present in an amount in excess of its solubility in thebath and providing an undissolved residue thereof in said bath and aportion of said total dissolved CrO3 equal to about 10 to'about 99% byweight of said total, said potassium dichromate also providing dissolvedpotassium to suppress the saturationV concentration of the dissolvedsilicouoride; the concentrations of said dissolved sulfate, said portionof dissolved CrO3, saidsuppressed silicouoride, and said'dissolvedpotassium being self-regulated; the balance of said total dissolved Cr03being supplied vas chromic anhydrideg'said temperature, said total andsaid portion of dissolved CrO3, and said chromic anhydride varyingsubstantially in the manner defined by the area ABCDKIA of the graphshown in the accompanying drawing; and the ratio of total dissolved CrO3to the sum of dissolved sulfate plus V1/2 the dissolved silicouoridebeing below about 250: 1.

A11. The method of claim 10 wherein said CrO3/ catalyst ratio is variedby adding to the bath a compound more soluble therein than saidstrontium sulfate and containing Va soluble Yionic component ofstrontium sulfate, the amount of said compound being such as to maintainthe total dissolved sulfate concentration of the bath both saturated andself-regulated.

12. The method of claim 11 wherein said CrO3/ catalyst ratio isdecreased by adding a soluble sulfate compound t-o the bath to Vincreasethe total dissolved sulfate concentration.

13. The method of claim 1l wherein said CrO3/catalys ratio is increasedby adding a soluble strontium compound to the bath to suppress thedissolved sulfate concentration.

References Cited in the le of this patent UNITED STATES PATENTS2,640,021 Passal May 26, 1953 2,640,022 Stareck May 26, 1953 2,686,756Stareck et al. Aug. 17, 1954 2.787,588 Stareck et al. Apr. 2. 1957

1. AN IMPROVED METHOD OF ELECTRODEPOSITING CHROMIUM ON A METAL ARTICLEBY MEANS OF AN AQUEOUS CRO3-CONTAINING, CATALYST-CONTAINING, CHROMIUMPLATING BATH, SAID METHOD BEING CHARACTERIZED BY HAVING A CRO3/CATALYSTRATIO IN SAID BATH WHICH HAS A SUBSTANTIALY REDUCED TENDENCY TO CHANGEWITH CHANGES IN CRO3 AND CATALYST CONCENTRATIONS AND TEMPERATURES, WHICHCOMPRISES ESSENTIALLY: PASSING CURRENT FROM AN ANODE TO SAID ARTICLE ASA CATHODE IMMERSED IN SAID BATH AT A TEMPERATURE OF 90 TO 140*F., SAIDBATH CONTAINING A TOTAL OF DISSOLVED CRO3 OF ABOUT 150 TO ABOUT 380G./1., STRONTIUM SULFATE AND POTASSIUM SILICOFLUORIDE ASCATALYST-SUPPLYING COMPOUNDS EACH IN AN AMOUNT SUFFICIENT TO SATURATESAID BATH WITH DISSOLVED CATALYTIC SULFATE AND SILICOFLUORIDE,RESPECTIVELY, AND TO PROVIDE THEREIN AN UNDISSOLVED RESIDUE OF EACHCOMPOUND, AND POTASSIUM DICHROMATE AS A NON-CATALYTIC, SILICOFLUORIDE-SUPPRESSING, CRO3- AND POTASSIUM-SUPPLYING COMPOUND, SAID POTASSIUMDICHROMATE BEING PRESENT IN AN AMOUNT IN EXCESS OF ITS SOLUBILITY IN THEBATH AND PROVIDING: (1) AN UNDISSOLVED RESIDUE THEREOF IN SAID BATH, (2)A PORTION OF SAID TOTAL DISSOLVED CRO3 EQUAL TO ABOUT 30 TO ABOUT 77 BYWEIGHT OF SAID TOTAL, AND (3) AN AMOUNT OF DISSOLVED POTASSIUM TOSUPPRESS THE CONCENTRATION OF THE DISSOLVED SILICOFLUORIDE, THECONCENTRATION OF SAID PORTION OF DISSOLVED CRO3 BEING SELF-REGULATEDOWING TO THE PRESENCE OF SAID DISSOLVED RESIDUE OF POTASSIUM DICHROMATE,SAID SUPPRESSED SILICOFLUORIDE CONCENTRATION BEING SELFREGULATE OWING TOTHE PRESENCE OF SAID UNDISSOLVED RESIDUE OF POTASSIUM SILICOFLUORIDE,SAID DISSOLVED POTASSIUM CONCENTRATION BEING SELF-REGULATED BY VIRTUE OFTHE PRESENCE OF SAID UNDISSOLVED RESIDUES OF POTASSIUM SILICOFLUORIDEAND POTASSIUM DICHROMATE, AND SAID DISSOLVED SULFATE CONCENTRATION BEINGSELF-REGULATED OWING TO THE PRESENCE OF SAID UNDISSOLVED RESIDUE OFSTRONTIUM SULFATE, 70 TO 170 G./1. OF SAID TOTAL DISSOLVED CRO3 BEINGSUPPLIED AS CHROMIC ANHYDRIDE, THE RELATIONSHIP BETWEEN SAIDTEMPERATURE, SAID CHROMIC ANHYDRIDE, SAID TOTAL AND SAID PORTION OFDISSOLVED CRO3 BEING VARIABLE IN THE MANNER DEFINED BY THE AREA EFGHE OFTHE GRAPH SHOWN IN THE ACCOMPANYING DRAWING, AND THE RATIO OF TOTALDISSOLVED CRO3 TO THE SUM OF DISSOLVED SULFATE PLUS 1/2 THE DISSOLVEDSILICOFLUORIDE BEING IN THE RANGE OF 115:1 TO 140:1.